The present invention relates to the recycling of spent tire curing bladders, which essentially contain cured butyl rubber, by grinding the bladders into fine size particles for use as a filler in rubber compositions.
Heretofore, spent or old tire bladders used in shaping the internal portion of a tire in a curing mold have generally been scrapped and discarded as a solid waste material in a landfill. Some of the tire curing bladders have been recycled by cutting the same in the form of a shoe sole, a beach tong, and the like.
A rubber blend composition suitable for use in a tire as for the innerliner thereof is made by mixing rubber compounds such as halobutyl rubber containing additives therein with fine size particles derived from grinding old, spent tire curing bladders. The blend when used as an innerliner for a tire has good physical properties such as air retention.
Curing bladders are utilized for making tires for off-the-road vehicles such as tractors, earth-moving equipment, and the like, as well as for on the road vehicles such as passenger cars, trucks, buses, and the like. Curing bladders are typically made from butyl rubber and contain conventional amounts of various rubber additives such as carbon black, oil, zinc oxide, generally phenolic curing aids and optionally small amounts of sulfur and sulfur accelerators. The amount of the various additives will vary from manufacturer to manufacturer, but generally include from about 20 to about 100 and desirably from about 40 to about 65 parts by weight of carbon black for every 100 parts by weight of butyl rubber (PHR), from about 2 to about 20 and desirably from about 5 to about 15 parts by weight of oil PHR, from about 3 to about 15 parts by weight of phenolic curing resins PHR, and very small amounts of zinc oxide such as from about 1.0 to about 8 parts by weight PHR.
It is an important aspect of the present invention that the generally spent, old, or used cured curing bladders are ground into fine size particles. The particle size according to U.S. Standard Mesh is generally from about 30 mesh to about 200 mesh and desirably smaller than 60 mesh such as from about 60 mesh to about 100 mesh. Generally any rubber grinding method or process can be utilized so long as the rubber is not scorched or degraded during grinding thereof. One particular method of grinding involves grinding the rubber in the presence of water, which keeps the rubber temperature low to prevent reversion. A more detailed description of such a preferred grinding method is set forth in U.S. Pat. Nos. 4,374,573; 4,714,201; 5,238,194; and 5,411,215, which are hereby fully incorporated by reference. Cryogenically ground rubber can also be utilized.
While the innerliner rubber can contain butyl rubber, preferably it contains halobutyl rubbers such as chlorobutyl or bromobutyl rubber. More preferably, the innerliner rubber is predominately bromobutyl rubber with optionally small amounts of natural rubber therein as from about 0 or 0.1 to about 50 percent by weight and desirably about 5 to about 15% by weight based upon the total weight of the butyl rubber and the natural rubber (PHR). The innerliner rubber can contain various conventional or typical innerliner additives such as carbon black; oil; curing aids such as sulfur, sulfur-containing compounds, and the like; vulcanizing accelerators such as amines, disulfides, guanidines, thioureas, thiazoles, thiurams, sulfenamides, dithio-carbamates, and the like; various anti-oxidants such as phenylene di-amines; various antiozonates; various aliphatic acids such as stearic acid; zinc oxide; various waxes such as micro-crystalline wax; various peptizers; various retarders, various resins, various fillers such as clays; silica; silica coupling agents; and the like. While the amount of the various additives will vary from one tire manufacturer to another, they generally include from about 20 to about 100 and desirably from about 50 to about 90 parts by weight of carbon black PHR, from about 5 to about 20 parts by weight of oil PHR, from about 0.5 to about 4.0 parts by weight of stearic acid PHR, from about 0.5 to about 8 parts by weight of zinc oxide PHR, from about 0.1 to about 3.0 parts by weight of sulfur PHR, from about 0.5 to about 5.0 parts by weight of sulfur accelerators, and the like.
According to the present invention, from about 1 to about 60 parts by weight and desirably from about 2 to about 20 parts by weight of ground cured curing bladder rubber particles are added to 100 parts by weight of an uncured innerliner rubber and blended in any conventional or typical manner. Suitable mixing methods include blending in a Banbury, mixing on a two-roll mill, or the like. More specifically, the fine ground cured curing bladder or rubber particles containing additives therein are added to butyl and/or halobutyl polymers, carbon black, stearic acid, oils and resins in a Banbury mixer and mixed to form an innerliner master batch. Zinc oxide, sulfur, and accelerators are then added to the master batch and blended as in a Banbury mixer to form the blended rubber composition of the present invention. The blended innerliner rubber compound-cured curing bladder particles are processed in a calender to form sheets of any desired width and thickness. In the assembly of a tire, the sheeted rubber innerliner-curing bladder rubber particle blend is applied to a tire assembly machine and the various other tire components applied thereto. The tire is then formed into its final shape and cured in a curing press.
The present invention is environmentally friendly inasmuch as spent or old curing bladders do not end up in landfills but rather are recycled into useful products such as a tire innerliner. When utilized as a tire innerliner, good retention, or substantially no loss of properties such as high air impermeability, is obtained. The utilization of the ground curing bladder rubber particles can also result in a cost saving in the preparation of a tire innerliner.